Preheat oven to 180°C. Spray a 23cm square ovenproof dish with olive oil.
Cut bread into 2cm cubes and place in a bowl with milk. Stand for 10 minutes. Spray a non-stick frying pan with oil. Cook onion and mushrooms for 3–5 minutes on medium, until tender and liquid has evaporated.
Combine mushroom mixture in a large bowl with bread cubes, spinach and tomatoes. Spoon into prepared pan. Pour over combined eggs and eggwhites. Bake for 35 minutes, until set.